Solid state power electronics are used in conjunction with high voltage transmission lines to improve system performance. Operations to improve system performance include power factor correction, voltage regulation, and resonance damping. These operations are executed as solid state power electronic devices rapidly switch high voltage capacitors and inductors. This switching operation is realized by several thyristor power semiconductors connected in series so as to withstand the maximum expected line voltage. Each thyristor is switched on by injecting a current into its gate-cathode junction. In a series string of thyristors supporting a large voltage at the time of switching, all thyristors must conduct simultaneously to prevent overvoltage destruction of the slower thyristors.
Thyristor activation is commonly achieved with a voltage isolation transformer with a winding operating on the high-voltage side of the power electronic device. The current gate drive signal is then produced with a resistor positioned between the thyristors and the secondary side of the voltage isolation transformer. There are a number of problems with this voltage source gate drive arrangement. First, it is difficult to obtain uniform gate pulses at each thyristor because of the normal variations in thyristor gate impedances, and because one or more thyristors may fail, thereby resulting in a shorted gate. Second, the magnitude and other parameters of voltage-sourced pulse trains are controlled at the high voltage-side of the system. Consequently, each thyristor may require costly and complicated equipment such as a power supply, inverter, and feedback loop for current control. Third, isolation of the high-voltage thyristor circuit from the low voltage control circuit is costly and bulky. It would be highly desirable to develop a thyristor gate drive device that provides uniform gate pulses, regardless of variations in thyristor impedance or the presence of failed thyristors. It would also be desirable to develop a thyristor gate drive device that operates at low voltage and thereby eliminates the requirement for costly and complicated high-voltage side control devices. Naturally, if a low voltage-side thyristor gate drive device is to be used, it should have a simple and inexpensive isolation mechanism from the high-voltage side of the power conditioning apparatus.
Current pulse shaping circuits for thyristors are activated with edge-triggered logic. Edge-triggered logic is susceptible to activation by noise spikes, cross-talk, and other transients. If an edge-triggered logic device erroneously fires a thyristor, a large differential voltage may exist across the switch at the point of firing. For instance, in the case of a thyristor switched capacitor, the capacitor may be negatively charged to the negative-peak line voltage and then be switched on during the positive-peak line voltage. In this case, the capacitor will encounter a rapid voltage change. This results in extremely high currents which can damage the thyristors and the capacitor.
In addition to the problem of high current due to misfiring, the inductance of the power line and the capacitance from the capacitor form an LC circuit with a step input of twice the peak line voltage. The capacitor will therefore encounter peak voltages of up to 3 times the nominal peak line voltage, which can eventually cause the capacitor itself to fail. Since failed capacitors tend to form short circuits, the associated thyristors may then have to interrupt the full fault current of the line, which can destroy the thyristors.
The problem of false triggering of edge-triggered logic has previously been addressed with shielding of critical signal lines and by minimizing imperfections in grounding. Unfortunately, these efforts have only incrementally improved the false firing problem and thereby have not substantially eliminated the phenomenon. Therefore, it would be highly desirable to provide a technique for eliminating the problem of false thyristor gate firing signals.